Acetylcholine (ACh) is one of the most important neurotransmitters in the peripheral and central nervous systems. However, it is increasingly recognized that ACh is also a key molecule for signaling via nicotinic acetylcholine receptors (nAChRs) in non-neuronal cells. Mounting evidence indicates that subtypes of ?9-containing nAChRs are critical receptors of ACh in these non-neuronal cells. These receptors are implicated in a plethora of functions including immune function, cell migration and the stress response. Consequently, these nAChRs have been implicated in diseases ranging from chronic pain to cancer-cell proliferation. Unfortunately, study of these ?9-containing nAChRs is severely limited because of the lack of selective ligands. There are no ligands selective for human ?9-containing nAChRs. Furthermore, existing ligands are unable to discriminate among ?9-containing nAChR subtypes in any tested species. To address this problem, conotoxins that are antagonists of ?9-containing nAChRs will be exploited. Aim 1 will develop selective antagonists for the human ?9 nAChR. This will be accomplished through iterative synthesis of analogs of ?-conotoxin RgIA and by development of newly discovered ?-conotoxins. Aim 2 will characterize, for the first time, new families of conotoxins that target ?9-containing nAChRs. These will be developed to enable the selective block of subtypes of these nAChRs. A long-term goal is to use existing ligands, together with newly developed toxins to gain mechanistic insight into the hypothesized role of ?9-containing nAChRs in preventing chronic pain that ensues following nerve injury. Aim 3 will use developed ligands to study the role of block of ?9-containing nAChRs in models of breast cancer.